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Old 6th April 2004, 03:47 PM   #1
piewie is offline piewie  Belgium
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Angry Aleph 5 100Hz problem

Hi,
i have just completed both monoblocks of my Aleph 5 each consisting of: -a 750VA +-30V toroid
-300.000uF capacitors
-a thermistor
-a net filter
-two 25A diode bridges
-regular aleph5 circuit
-star ground

The amplifiers both play and draw about 1.2A at 230V so no problem there. But... at the output there is some 100Hz AC. (according to my fluke)
Its about 10mV in size and it looks like a small top of a sine at 1/5th or so of a period. So only the positive tip is visible and only at one fifth of a period.
The DC ofset is 15mV and 20mV so that should be no problem either.
But the question is, what is this distortion on my output? And more important, how can i get rid of it?

Any ideas?
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Old 6th April 2004, 04:25 PM   #2
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Holger Barske's Avatar
 
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This is definitely a rectified 50 Hz issue, kind of a classic .
You have some kind of grounding problem. Could you post some pics of the amp's inside?
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Old 6th April 2004, 04:52 PM   #3
piewie is offline piewie  Belgium
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I'm uploading pictures right now.
In the meantime i'll give a short explanation on ground connections made in the case.

starground is made of a m4 steel bar on which all the connections are made, separated by a bolt.
Connections: incoming ground from ac plug
chassis (bolt is fixed at chassis)
gnd from upper cap bank
gnd from lower cap bank
gnd from pcb
gnd from speaker protection
2 gnds from the two rectifier bridges
gnd from supply filter

The gnds from signal wire only connect at pcb.
Two capacitor banks are paralleled and connected to the pcb.

Things i've tried allready:
disconnect gnd bar from chassis.
disconnect gnd bar from chassis, remove ac gnd from it and connect the ac gnd to chassis.
removed the input cables.
disconnected ac gnd.

Didn't change the noise a bit

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

output distortion:

Click the image to open in full size.
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Old 6th April 2004, 05:48 PM   #4
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Alain Dupont's Avatar
 
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Location: Paris - France
piewie,

1)- Remove the protection circuit, and check for the noise...

2)- Twist the cables for AC current going to the rectifiers, and twist DC rectifiers out together... that should help for rejection...

3)- How is the big black relay wired, in DC, with a separated supply...?

4)- Check for the noise all along, starting from the rectifiers, just at the first capacitor and then after the last capacitors, for each channel supply
and tell us what are the values you measured. Without the channels connected
{in this case load the supply's with a 10 Ohms 50 watts resistor}
Then with 1 channel, then the other channel alone, and finaly the 2 connected...

Regards.

Alain.
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Old 6th April 2004, 06:06 PM   #5
piewie is offline piewie  Belgium
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removal of the proctection circuit changes nothing.

twisting the wires : i'll try that later, but they are only 15cm long
and go directly to the contactor. from there on to the trafo
trafo directly to the bridge rectifiers (this connection cannot be twisted, solid 2mm copper). Every connection is with 2.5mm˛ and sleeve connectors.

Contactor wiring: separate black transformer to a supply print which
feeds the contactor 24V via a start stop circuit.

Noise on rectifiers and capacitor banks is the same: sawtooth of about 200mv AC at +- 38V DC
strange thing: gnd pin at rectifiers gives same distortion figure (mirrored for the other rectifier bridge)
output distortion is everywhere after the fets
sawtooth is on every input supply.
Amp is loaded with a normal 8 ohm test speaker (amp works but humms)
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Old 6th April 2004, 06:20 PM   #6
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piewie,

I'll download the Aleph 5 schema after work, and will come back.

Regards.

Alain, at work...

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Old 6th April 2004, 06:52 PM   #7
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piewie,

If the power supply's are Ok then ten amp circuit:

1)- If you had a good power supply 2*35 volts ; I would recommend making a test with it before further research...

2)- Take a look at the CCS of Q3: Zener Z5, take a scope to see how
is the voltage reference there.

Regards.

Alain.
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Old 6th April 2004, 07:02 PM   #8
piewie is offline piewie  Belgium
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Hi, power supply was ok yes.

I think i have located the problem.
The peak currents are rather large for a pcb trace.
Because the connector for the pcb supply was rather close to
the connector of the raw rectified voltage, current chose the easiest way and skipped mostly the capacitors. I have now placed the intake on the upper front, and the regulated out on the lower back of the capacitor bank. Now current is forced trough the entire bank and the distortion seems disappeared at a first test.
I'll do some more tests and give you an update.
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Old 6th April 2004, 07:08 PM   #9
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piewie,

Seems a good fix. Good luck...

Alain.
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Old 6th April 2004, 07:16 PM   #10
piewie is offline piewie  Belgium
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ok, seems like the problem is fixed.
distortions seems disappeared at the output (i only have a x10 probe ) and it looks like the capacitor ripple has decreased to.

Now noise is only noticable at 5cm of the speakers, makes quite a difference of the humm which bothered me at two meters distance.

Thank you all guys for the efforts. I'll update the second monoblock in a minute and then i'll listen to some music.
I'm looking forward to be able to listen to and to enjoy my diy Aleph.
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